You need the Par level of the fixture you want. Plants can use light from about 5000K to 10000K. I like 6000-6700K as they make red/pink fish and plants look better but many find that spectrum too dim for their liking. People often use 10K because it is very bright to the human eye but it washes out the color of red/pink fish and plants. BML fixtures actually makes a 10K that doesn't wash reds out. The type of plants you want to grow will determine the Par level you need to be looking at. Also remember medium to high Par levels in a tank means you will need to use higher doses of liquid carbon or pressurized carbon and a good fert regime. In a large tank dry ferts are very economical. You need to consider a lot of things before choosing which fixture you want to use.